The invention relates to an image forming apparatus such as a copying machine, a printer, and a composite machine which has a copying function and a printing function, and an optical multi-beam scanning device which is mounted on the image forming apparatus. Specifically, the invention relates to the optical multi-beam scanning device and the image forming apparatus for a color mode.
Image forming apparatuses mainly adopt a method in which a number of light beams to be scanned at a time is increased (multi-beam) as speed heightening means of writing optical systems. In writing optical systems of four-tandem (one pass) color composite machines, it becomes mainstream that a polygon mirror, a scanning lens, and the like are shared in order to reduce the cost.
In the case where the adoption of multi-beam is considered, a method of synthesizing laser beams from individual LDs (laser diodes) with one another using a beam splitter, a half mirror and the like so as to lead the synthesized laser beam to a polygon mirror, and a method of leading a plurality of laser beams to a polygon mirror using an LD array for emitting the laser beams to one direction are considered. From a viewpoint of an installation space, the LD array is advantageous, but when a number of scanning light beams increases to 4, 8, . . . , synthesization using the beam splitter should be used at the same time. In the four-tandem (1 pass) color composite machines, light beams (laser beams) should be separated by a mirror so as to go to photosensitive drums for respective color components after deflection. Since separation intervals are too narrow in the method using the LD array, the method using the individual LDs is preferable.
There are a lot of optical multi-beam scanning devices which adopt the method of synthesizing laser beams from the LDs using the beam splitter, the half mirror, and the like so as to lead the synthesized laser beam to the polygon mirror.
Optical parts for switching a ratio of reflection and transmission by a deflecting direction of a beam from the beam splitter, the half mirror and the like or by the thickness of a reflecting layer, are applied to a synthesizing unit for optical paths of the laser beams. Even in this case, a laser beam (excessive light), which moves to a direction (for example, a reflecting direction) other than an intended direction (for example, a transmitting direction), is also present, and this laser beam becomes stray light and it might deteriorate the performance of the apparatus.
A technique relating to an optical pickup is disclosed in Japanese Unexamined Patent Application Publication No. 5-54421. In this publication, a taper portion of 45° is provided on a portion for housing optical parts of the optical pickup, and excessive light emitted from a polarized light beam splitter is reflected by the taper portion (an optical path of the excessive light is folded by 90°) so as to let it go to the outside. There is also described that if the taper portion is painted black, it is more effective.
Such a stray light countermeasure against the application of the optical parts such as the beam splitter and the half mirror can be applied only to the case where rear positions of stray light emitting surfaces of the optical parts have a sufficient space. In the optical multi-beam scanning device which leads a plurality of laser beams from a plurality of LDs to one polygon mirror, since installation positions of the respective optical parts are close to one another, the above method cannot be applied in most cases from the viewpoint of the space.